The present invention relates to dielectric ceramic compositions allowing low-temperature sintering, and exhibiting a high dielectric constant, low temperature coefficient of the dielectric constant, and low dielectric loss, and being suitable for use in ceramic capacitors.
As promising ceramic compositions for this purpose those essentially consisting of BaTiO.sub.3 have been widely used as materials of high dielectric constant. The dielectric constant of this material relates to its temperature coefficient. A dielectric material which exhibits a high dielectric constant, has a relatively high temperature coefficient of the dielectric constant, and a dielectric material which exhibits a low dielectric constant has a relatively low temperature coefficient of the dielectric constant.
The temperature coefficient of a ceramic capacitor is established by JIS (Japanese Industrial Standard) or EIA (U.S. Electronics Industries Association) Standard. For example, the temperature coefficient of a dielectric constant adapted to YF rating of JIS, which means that the temperature coefficient lies within a range of +30 to -80% at an operating temperature of -25.degree. to 85.degree. C., is approximately equivalent to Y5V rating of EIA Standard which means that the temperature coefficient lies within a range of +22 to -82% at an operating range of -30.degree. to 85.degree. C. A dielectric material which exhibits a dielectric constant of 10,000, is adapted to YF or Y5V. The temperature coefficient of a dielectric constant adapted to YD rating of JIS, which means that the temperature coefficient lies within a range of +20 to -30% at an operating temperature of -25.degree. to 85.degree. C., is approximately equivalent to Y5T rating of EIA Standard which means that the temperature coefficient lies within a range of +22 to -33% at an operating range of -30.degree. to 85.degree. C. A dielectric material which exhibits a dielectric constant of 4,000 is adapted to YD or Y5T. However, BaTiO.sub.3 system ceramics must be sintered at a very high temperature in the range of 1300.degree. to 1400.degree. C., so that where they are used as a dielectric material of multilayer ceramic capacitors, expensive metals such as platinum or palladium which can stand such high sintering temperature should be used as internal electrodes of the capacitors. Therefore there has still been a demand for dielectric ceramic materials which can be sintered at a temperature as low as below 1000.degree. C. for enabling the use of relatively cheap metal such as silver for the internal electrodes.
U.S. Pat. No. 4,078,938 discloses binary system ceramic compositions of PbFe.sub.2/3 W.sub.1/3 O.sub.3 --PbFe.sub.1/2 Nb.sub.1/2 O.sub.3 which can be sintered at a relatively low temperature, and which exhibit a high dielectric constant of 20,000. However, the ceramics have a high temperature coefficient of the dielectric constant, which is adapted to only YF rating of JIS or Y5V rating of EIA Standard.
U.S. Pat. No. 4,236,928 discloses binary system ceramic compositions of Pb(Fe.sub.2/3 W.sub.1/3)O.sub.3 -Pb(Zn.sub.1/2 Nb.sub.1/2)O.sub.3 which can be sintered at a temperature below 1000.degree. C., and which exhibit a dielectric constant of 5000. However, the ceramics also have a high temperature coefficient of the dielelctric constant which is adapted to a Z5U rating of EIA Standard which means that the temperature coefficient lies within a range of +22 to -56% at an operating range of +10.degree. to +85.degree. C.